The couple of progressions I used to achieve good pistols were
1.) Extend your non-standing leg at as close to parallel to the ground as possible. Start to lower into a pistol from the standing position with a table, bed or other surface on the same side as the standing leg. When balance or strength start to challenge the rep use your hand to provide as little assistance as possible - preferably just a light touch to give some balance and confidence, a minor push if necessary. Aim to descend as low as possible, "ass to grass" being the goal, right? Settle gently into bottom position, preferably pausing there so that you don't "bounce out" of the bottom and cause undue stress on your knee. Reverse this to stand.
2.) Stand on a stable, raised surface such as a park bench, cement parking barrier. Lower on one leg with the other leg hanging off of the raised surface. This allows you to focus less on keeping the "off" leg rigid and more on the pushing force in the squatting leg.
While practicing both of these I found it important to A.) Concentrate on keeping weight mostly in my heels (really distributed through the whole foot, but this will feel like "all heel" to most of us who tend to stay in the balls of our feet), back straight and settling back, leading with the butt as if sitting in a chair. B.) Practice GOOD reps. All of these are going to feel challenging while learning and developing a good, clean pistol. It's true. Get a feel for how good you are when you start a session and if after a few reps you find you have deteriorated, refocus your effort on a different movement. These are very skill and nervous system intense. "Getting them right" is more a matter of fine tuning than big muscular strength.
I hope these are somewhat helpful. For really good discussion of pistols (as well as one arm push ups) maybe you can get your hands on a copy of Pavel Tsatsouline's "The Naked Warrior"?
